(1) Field of the Invention
The present invention relates to a flexible wiring board for tape carrier package having improved flame resistance, particularly that comprising an overcoat layer having an oxygen index exceeding 22.0; as well as to a tape carrier package formed using such a flexible wiring board.
(2) Background Art
“Tape carrier package” is a package obtained by mounting electronic part(s) (e.g. a semiconductor chip) by, for example, TAB (tape automated bonding) technique on a flexible wiring board which is a tape carrier comprising an insulating film and a metal wiring pattern formed thereon.
A flexible wiring board, typically a tape carrier for TAB application, is manufactured by the following method. In an insulating film with an adhesive layer, having a width of, for example, 35 mm, 48 mm or 75 mm, there are formed, by press-cutting, perforation holes (sprocket holes) for tape transfer; device holes for forming inner leads to which a semiconductor chip is to be bonded; bending slits at which the wiring board is bent when used; etc. To the resulting insulating film with an adhesive layer is heat-bonded an electro-conductive metal foil (e.g. a copper foil). Then, a flex resin layer is formed on each bending slit portion (the back side) of the metal foil, for protection of the metal foil; thereafter, a photosensitive resin resist is coated on the metal foil, followed by exposure, development and etching to form a wiring pattern on the insulting film. The wiring pattern crosses each bending slit, forms inner leads at each device hole, and forms outer leads for bonding with other parts, at the pattern end. Next, the surface of the most part of the wiring pattern-formed region is protected by an overcoat layer; and the surface of the wring pattern portion having no protecting film formed thereon (this portion includes connecting portions of the wiring pattern, such as inner leads and outer leads) is surface-treated by thin plating, gold plating, solder plating or the like.
In this flexible wiring board for tape carrier package, each terminal of a semiconductor chip is bonded to each inner lead by means of bump or the like, whereby the semiconductor chip is mounted; and the semiconductor chip is covered with and protected by a semiconductor-encapsulating resin to obtain a tape carrier package. This tape carrier package is produced in such a form that same electronic part units each comprising an insulating film, a wiring pattern formed thereon and a semiconductor chip or the like mounted on the wiring pattern are arranged repeatedly in a continuously tape form. The tape carrier package having a continuous tape form is cut into individual electronic part units; the outer leads of each electronic part are bonded with other parts; as necessary, bending is made at the bending slit portion(s); and the resulting material is incorporated into or mounted on an electronic apparatus. For example, in a liquid crystal display, a tape carrier package having a driver LSI of the display thereon is mounted in a state that bending has been made at the bending slit portion(s).
In conventional flexible wiring boards for tape carrier package, there have been preferably used a heat-resistant aromatic polyimide film as the insulating film, a modified epoxy resin as the adhesive layer, a cured material made from a polyimidesiloxane resin composition (which is flexible, has an appropriate mechanical strength, is superior in electrical insulation and has heat resistance), as the flex resin layer, and a cured material made from a polyurethane resin composition, as the overcoat layer.
Tape carrier packages using such a conventional flexible wiring board have been used mainly in a liquid crystal display operating with a low voltage. Therefore, there has been no particular problem with respect to the flame resistance. Recently, however, it is in progress to use a tape carrier package in electric or electronic apparatuses operating with a high voltage, such as PDP (plasma display panel) and the like; in that case, it has been necessary to improve the flame resistance of the tape carrier package and the flexible wiring board used therein, for prevention of fire outbreak.
JP-A-1999-121682 and JP-A-1999-220248 describes a flexible wiring board wherein each bending slit portion is insulated/covered with a flex resin (a solder resist) having particular mechanical properties. Also JP-A-2004-211064 discloses a polyimidesiloxane composition for formation of a cured insulating film of electronic part. In these prior arts, however, there is no description on the flame resistance of flexible wiring board for tape carrier package, and there is neither description nor suggestion particularly on a laminate of flex resin layer, insulating film, adhesive layer and overcoat layer, as well as on the improvement of flame resistance thereof.